This invention relates to a method for contactless evaluation of the characteristics of semiconductor wafers and devices, such as evaluation of surface contamination by heavy metals or other contaminants, bulk contamination because of diffusion of the contaminants, mechanical surface damage, subsurface crystal defects, or damage by ion implantation. This invention more specifically relates to an improvement of contactless measurement of photoinduced carrier lifetime and surface recombination velocity of the semiconductor wafers made of silicon.
For evaluation of characteristics of semiconductor wafer, measurement of carrier lifetime is advantageous. Several contactless methods for the lifetime measurement have been developed, such as diffusion length measurement, the photocurrent method, and the widely used photoconductivity decay method. The photoconductivity decay method has become the technique most used to characterize the ingot material, etc. The present inventors have proposed various inventions relating to a photoconductivity decay method, which will be described hereinafter.
An example of the photoconductivity decay method was proposed in Japanese Patent Application Examined (kokoku) No. 61-60576. The method is as follows:
(1) A microwave generator continuously irradiates microwaves via a wave guide to the surface of a wafer.
(2) A pulsed light source applies a light pulse to the surface of the wafer, thereby exciting carries in the wafer.
(3) The generated excess carriers increase the conductivity of the wafer, thereby the reflected components of the incident microwaves are thereby modulated by the varying conductivity, e.g. the phase of the microwaves is effected.
(4) When the light pulse has ceased, the majority and minority carriers recombine to reach the equilibrium state. Over the time course of this reequilibration, the absorption of incident microwaves likewise returns to its pre-excitation state.
(5) As the above process occurs, a detector collects the modulated microwaves reflected from the surface of the wafer via the wave guide. The detector continuously transforms the changing phase of the reflected microwaves to an electrical signal and outputs the signal. An oscilloscope connected to the detector displays decay curve which graphically illustrates the time course of the electrical signal that shows the changing phase of the microwaves which represents the recombination time course of the minority and majority carriers.
(6) The carrier lifetime of the wafer can be computer-calculated by referring to the decay curve. In this case, carrier lifetime is calculated as time for the electrical signal of the detector to decay to 1/e wherein e is the base of the natural logarithm.
Next, Japanese Patent Application Examined (kokoku) No. 58-57907 will be described. In general, the actually measured lifetime .tau..sub.m is defined by formula (1): ##EQU1## where .tau..sub.b is bulk lifetime, mostly determined by crystalline perfection of the wafer, .tau..sub.s is surface lifetime determined by the surface condition of the wafer which may be effected by machining damage, crystal defects and surface contamination. Surface lifetime .tau..sub.s is inversely proportional to surface recombination velocity S. With higher surface recombination velocity S, induced excess carriers diffuse and recombine more rapidly at the surface, thereby measured lifetime .tau..sub.m is frequently too lower to be measured. To evaluate crystal characteristics of the wafer by measuring lifetime .tau..sub.m, it is necessary to reduce the surface recombination velocity S. Because in wafer-like semiconductors, large number of the induced excess carriers diffuse and recombine at the surface, a positive charge coating method which is disclosed in Japanese Patent Application Examined (kokoku) No. 58-57907 is now utilized in order to lower surface recombination velocity S. In this method, positive ions such as tin chloride are implanted into the surface of n-type semiconductor. The positive ions combine with induced electrons (minority carriers) to restrict the recombination of holes (majority carriers). Thereby, the surface recombination velocity S is lowered. Therefore, the measured lifetime .tau..sub.m may be regarded as the bulk lifetime .tau..sub.b. Also, a negative charge coating method is utilized in order to lower surface recombination velocity S. In this method, negative ions are implanted into the surface of p-type semiconductor. The negative ions combine with induced holes (minority carriers) to restrict the recombination of electrons (majority carriers). Thereby, the surface recombination velocity S is lowered. Therefore, the measured lifetime .tau..sub.m may be regarded as bulk lifetime .tau..sub.b. At present, precise heat annealing is achieved to make a regular oxide layer on the surface of the semiconductor wafer and device by the wafer and device manufacturers. A barrier of surface state is made at the boundary surface between the oxide layer and the bulk. Because the electrons and holes are charged at the boundary in P-type silicon and N-type silicon respectively, the surface recombination velocity S is constantly restricted to achieve more precise bulk lifetime measurement. The present lifetime measurement using microwave is utilized for examination of the effect of heat annealing or evaluating the operating condition of heat annealing furnaces. However, in both methods, the specimen which is used for measurement may not then be used for a wafer or semiconductor device.
Because in the above methods, the specimen which is used for measurement cannot later be used as a wafer or semiconductor device, Japanese Patent Application Examined (kokoku) No. 62-53944 was developed as technique for contactless measurement of bulk lifetime and surface recombination velocity of wafers in which the surface has been ground and which may be actually utilized as semiconductor material in that condition. Japanese Patent Application Examined No. 62-53944 discloses a method for calculation of bulk lifetime .tau..sub.b and surface-recombination velocity S by computer-analysis of lifetime .tau..sub.m measured after irradiation of the wafer with a light pulse generator. The method is as follows:
When excess carriers decay after irradiation by a light pulse, concentration-distribution of carriers is according to formula (2). ##EQU2## where D is the diffusion coefficient defined by the specimen material, .DELTA.p (x, t) is the excess carrier concentration, x is depth from the surface irradiated by light, and t is elapsed time after beginning the irradiating by the light pulse. The boundary condition of formula (2) is determined according to formulas (3) and (4). ##EQU3## where w is the thickness of the wafer. If these formulas are transformed and the measured lifetime .tau..sub.m is substituted, then the surface recombination velocity S and bulk lifetime .tau..sub.b may be calculated separately.
At present, as a method for evaluation of the characteristics of semiconductor wafer, a combination of Japanese Patent Applications Examined No. 61-60576 and 58-57907 which are for measurement of bulk lifetime .tau..sub.b is utilized. Also, Japanese Patent Application Examined No. 62-53944 which is for obtaining bulk lifetime .tau..sub.b and surface recombination velocity S after measurement of lifetime .tau..sub.m is utilized.
However, up to now, surface resolution of measurement for lifetime .tau. and/or surface recombination velocity S has been limited to no less than 2 mm by current methods, therefore accuracy in measurement is not adequate.
On the other hand, the method for separately calculating bulk lifetime .tau..sub.b and surface recombination velocity S disclosed in Japanese Patent Application Examined No. 62-53744 needs complex analysis to obtain results for a measuring point. Therefore, applying the method to many points over the wafer surface requires enormous time. Consequently, the method is not practical for "on-line" evaluation of wafers.
In recent years, semiconductor device manufacturing methods have become more precise, shallower areas must be joined with each other and function properly, and the semiconductor material is employed at a depth of only a few .mu.m from its surface. Thus, it is essential to evaluate the characteristics of these areas and to monitor surface conditions "on-line" before and during the semiconductor device manufacturing process. For example, junctions in 1 megabyte CMOS type DRAM are at a depth of 0.2 .mu.m. Therefore it is sufficient to measure the characteristics of shallower areas such as 2 or 3 .mu.m from surface, and measurement of deeper areas is not only unnecessary but undesirable.